Well, considering you can still play Runescape Classic, provided you met the requirements to do so in the past, I think Old School Runescape will be around for as long as Jagex is.
The main concern for Jagex should not be Runescape, it should be the very real fact that all of their projects post-Runescape have lost money or have been complete busts + scrapped projects. Mechscape, FunOrb, Stellar Dawn, all that stuff they were planning to do failed. Also, I don't see them making any sort of revenue off of that Transformers game.
Jagex needs to keep making money and to keep making Runescape better, but it's getting harder and harder for them to do so.